1. Field of the Invention
This invention relates to a precision positioning apparatus, and particularly to a precision positioning apparatus suitable for a positioning control system for a semiconductor exposing apparatus such as a stepper.
2. Related Background Art
In a precision positioning apparatus, and particularly the positioning of the wafer stage of a stepper, the positioning accuracy required is becoming higher with the recent and rapidly increasing degree of integration of semiconductive elements. The present day positioning accuracy at a practical level is on the order of several tens of nanometers for a stage carrying thereon a wafer to be positioned, but positioning accuracy on the order of one nanometer is expected to become necessary in the near future. In order to improve throughput, it is necessary to effect such highly accurate positioning at a higher speed.
Taking a stepper as an example, most of such positioning apparatuses according to the prior art have used a slide guide or a rolling guide as guide means for a stage which is a conveying body, and a system for converting the revolution of a DC servomotor into rectilinear movement through a ball screw has been used as driving means.
However, higher positioning accuracy results in a significant increase in friction and therefore, if a higher speed and higher accuracy are contrived by the use of such a contact type construction, the problem of long-term stability by frictional abrasion or frictional heat and the problems of unstable behavior such as a stick slip phenomenon and jumping phenomenon will arise and positioning accuracy on the order of one nanometer will be difficult.
So, in order to solve such problems, a stage of the non-contact driving type which uses an air bearing as a guide surface from its advantage of being free from the influence of contact friction and in which a linear motor having this air bearing as a guide is used as driving means has recently become the mainstream.
However, in a positioning apparatus using such an air bearing and such a linear motor, the rigidity in the direction of movement of the stage is lower than the rigidity by a contact type guide mechanism and a ball screw. Accordingly, a base plate and the stage operate as a kind of coupled vibration system of two degrees of freedom and therefore, vibration is caused between the stage and the guide surface by the rolling of the base plate resulting from the movement reaction force of the stage and the positioning time for the stage increases. Heretofore, use has chiefly been made of a base plate acceleration feedback method in which an acceleration sensor is attached to a base plate and the acceleration signal of the base plate detected thereby is added to the control input of a linear motor (see, for example, Hiroo Kinoshita, Munetsugu Kanai, Kokichi Deguchi and Tadao Saito (Kinoshita, et al.): "Air Bearing Guided High-Speed XY Stage", the Journal of the Precision Engineering Society, 52/10 (1986), p. 47 through 52.
Also, as compared with the prior-art contact type guide mechanism, the rigidity of the above-described apparatus is low and therefore, the disturbance of the postures such as yawing and pitching other than translation motion is great and such movement, like the vibration of the base plate, becomes the cause of the aggravation of positioning accuracy and an increased positioning time. However, the prior-art fixed table acceleration feedback has suffered from the problem that it can hardly correct such disturbance of the postures.